of/platform: Handle of_populate drivers in notifier

When using overlays with drivers calling of_populate the notifier
will try to create the device twice. Using the populated bit
before proceeding protects against this.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>